A power supply is generally used to convert commercially available alternating current power to direct current power for use by an ordinary electronic appliance. The conversion technique used in a power supply for a personal computer is based on the switching operation of switching devices to provide DC output voltage of multiple predetermined levels. This type of power supply is generally termed as switching regulator or switching mode power supply (SMPS).
However, the implication of power quality of a switching mode power supply always depends on its capability to exclude abrupt disturbance or maintain power conversion efficiency. A common problem that would cause the instability of the power quality of a power supply is due to voltage sag or voltage dip that is caused by an accident occurring between adjacent feed lines or voltage buses. The voltage sag generally means that the root-mean-square value (rms value) of voltage is dropped below its nominal value by 10% to 90%, and it would last for a prolonged period, for example, several seconds. The generation of voltage sag in an AC power source would greatly lessen the reliability of a power supply.
In a power supply system, brownout condition signifies a significant degradation of the primary power source. The danger of brownout condition is that it can repeat several times consecutively and is generally followed by a follow-on surge that can be several times of the voltage that a switching mode power supply is supposed to take in. It has been discovered in numerous brownout tests for switching mode power supply that in case the input voltage is abruptly dropped below its nominal value by about 50%, the output voltage is still under regulation. However, in case the input voltage is abruptly dropped below its nominal value by 80%, the output voltage will run out of regulation and start to bounce violently. The unstable output voltage generated by the power supply is likely to damage other electronic circuit powered by the power supply.
In addition to the brownout problem that is induced as the input voltage abruptly dropped below its nominal value by a predetermined amount, when the power supply is started up, a surge current may be induced at the input terminals of the power supply. Therefore the output voltage of power supply may overshoot upon start-up and sparks may generated. To limit the current surge from the input terminals of power supply upon start-up and provide a simple way to softly start the power supply, a device featuring with soft-start function is required to ensure a soft start-up of the power supply.
In view of foregoing problems, what is needed is an output voltage stabilizer that can stabilize the output voltage of power supply during brownout stage and/or soft-start process. The present invention can satisfy these needs.